The invention relates to an apparatus and method for monitoring an earth-leakage state of a power distribution system, and in particular of a DC traction system.
Traction systems exist, for example the London Underground rail system, which incorporate a pair of running rails for carrying the rolling stock and a pair of current rails (conductor rails) for supplying power to that rolling stock. Such a scheme is shown very schematically in FIGS. 1 and 2, in which a pair of running rails 10, 12 secured to sleepers 14 resting on ballast 19 (see FIG. 2) are associated with, in close proximity, a pair of current rails 16, 18 insulated from the sleepers by insulators 20. Conventionally, one of the current rails 18 is situated between the running rails, this being the negative DC power rail, and the other--the positive DC power rail 20--is situated outside the running rails.
FIG. 1 shows the power supply arrangement for the DC traction system, consisting of rectifiers 30 each fed from, typically, a 22 kV substation and outputting a DC voltage of 630 V which is fed to the current rails 16, 18 by way of circuit breakers 32. The entire stretch of line is divided into sections, FIG. 1 constituting one of said sections, and each section comprises a number of sub-sections 34 each fed at opposite ends from a pair of rectifiers 30. As illustrated in the diagram, sub-section 34A is supplied on its west side by a rectifier 30A and on its east side by a rectifier 30B, sub-section 34B is supplied on its west side by a rectifier 30B and on its east side by a rectifier 30C, and so on.
In addition, in order to maximise the voltage profile between substations, i.e. minimise I.sup.2 R losses in the system due to the very high supply currents (of the order of 4500 amps) needed to power the rolling stock in view of the low supply voltage employed, parallel tracks on the same section are also fed from the same respective rectifiers by their own circuit breakers, i.e. the current rails of the various parallel tracks are effectively connected in parallel. FIG. 1 shows the current rails 16', 18' of one such parallel track fed from circuit breakers 32'.
Neither of the current rails is directly connected to earth, the DC power distribution system therefore being a floating one (a so-called "isolee terre" ("IT") system), and there is a finite leakage resistance between the current rails 16, 18 (which are, by definition, uninsulated) and the running rails 10, 12 which are at earth potential. This resistance, under ideal service conditions, will be high--e.g. of the order of perhaps 100 k.OMEGA. per kilometre--and will arise, for example, from the finite resistance of the insulators 20, the normal surface condition of the insulators and sleepers and also the finite insulation resistance of the various electrical components on a train that may be standing at any particular instant on the tracks. In practice, however, the leakage resistance may be lowered due to either changed track conditions or a fault on a train present on the track.
Track-based faults may be relatively high-resistance faults due to damaged track insulators 20 or perhaps dampness, or low-resistance faults due to the shunting effect of an electrically conductive object that may have fallen onto the rails. Train-based faults are generally low-resistance faults arising from, for example, a low-slung negative shoe, which is designed to supply current to the train from the negative rail 18, accidentally making contact with a running rail when a set of points is reached, or an earth fault in the motor commutator arrangement of the train, whereby a brush may contact earth as the armature rotates.
This points up a general difference between track-based faults and train-based faults, namely that track-based faults tend to be steady faults giving rise to a constant shift in leakage resistance between the relevant current conductor and earth, whereas the train-based faults described above create by their very nature an oscillatory shift in leakage resistance.
Under no-fault conditions, the relative potentials of the current conductors 16, 18 and earth (the running rails 20) are as shown in FIG. 2, i.e. the positive conductor 16 settles at +420 V and the negative conductor 18 at -210 V relative to earth. The non-symmetrical distribution arises from the construction of the current rails and insulators in relation to the running rails.
When a fault as described earlier occurs as a first fault on a track section or sub-section, the effect is merely to shift the relative potentials between the current rails and the running rails away from their no-fault values, and in an extreme fault condition one of the current rails may be effectively at earth potential leaving the other to carry the full 630 V relative to earth. This by itself is not injurious to the electrical supply equipment since the rectifier outputs merely float with respect to earth, neither does it represent a danger to train personnel or passengers (though it may represent an increased risk to line personnel who will be nominally at earth potential), since the train itself will be at earth potential. However, what does represent a risk to train occupants is the occurrence after the appearance of the first fault of a second fault, this time between the opposite current conductor and earth and arising from a fault on the train. When this occurs there is a low-resistance path between the two current conductors which can lead to severe arcing and heat and smoke generation in equipment located on the train, passengers' lives being thereby put at risk.
Subdivision of a track system into electrically isolated sections has been one attempt to reduce the risk of the occurrence of such a second, opposite-pole, fault, but there has remained the need to detect a first fault state as quickly as possible after its occurrence so that it could be located and remedied before a second, opposite-pole fault occurred.
One well established way of monitoring an earth fault is illustrated in FIG. 3, in which bleed resistors 40 and 42 are connected between the positive and negative conductors, respectively, and earth. Resistor 40 is conventionally 200 .OMEGA. and resistor 42 100 .OMEGA.. Connected across resistor 42 is a chart recorder 44 which records changes in voltage across resistor 42. Bleed resistors of low resistance value are used to swamp the actual leakage resistance between the various points and provide a predictable 2:1 voltage ratio for operating the chart recorder within nominal limits. Two such monitors are employed in each sub-section 34, one driving the chart recorder, as illustrated, in a power control room, and the other driving a lamp as a visual alarm in a line control room.
There are severe drawbacks associated with this known monitoring method. In the first place, the low-value resistors 40, 42 dissipate much power (approximately 1.3 kW total); thus, in a typical rail system comprising of the order of 100 sub-sections, there will be a total power dissipation of 100.times.2.times.2.times.1.3=520 kW. Secondly, because of the low resistances used it is impossible to detect slight changes in leakage resistance resulting from the imposition of a relatively high-resistance leakage shunt path between a conductor and earth. Thus, this system masks the effect of, for example, deteriorated insulation between one of the conductors and earth. A third drawback is that, because the chart recorder 44 measures a voltage which is set by a ratio, the ratio of positive rail to earth and negative rail to earth, it cannot sense the presence of a leakage source which affects both conductors equally. An example of such a source is permanent dampness in the ballast, or flooding.
A second scheme supplements the above bleed resistor arrangement with a so-called "unbalanced current protection" arrangement in which current detectors are installed around each substation to monitor track feeder cables, relays and so forth. When two opposite-pole earth faults occur, the flow of currents in the substation become unbalanced and this imbalance is sensed by the protection arrangement and a signal is sent to trip circuit breakers at the adjacent substation which then isolate the offending sub-section. However, this protection arrangement lacks the necessary sensitivity and stability to detect single-pole faults down to earth and requires the complete isolation of sub-sections in order to arrive at definable current flows for reliable decision-making. A known system employing such current monitoring and detection of residual, fault-caused currents is described in published British patent application GB 2,247,119A.